The big things I've encountered, just for another perspective, with 2-4 people on a hosted MP game (not dedicated):
- that melee reach that was mentioned (zombies hitting you from either very far away, or off to the side by 90°)
- the inability to open containers at all, let alone quickly, especially during horde night (which is inconvenient, to say the least)
- player positions being very far off (several meters minimum) between one person's client and another
- zombies not registering hits from firearms, and sometimes not from melee, sometimes because they're already dead on the host's machine
- zombies running in place for 1-3 seconds, then teleporting to a new location and running in place there - this makes them much harder to hit with anything, and often fails to register the hit